The above-referenced related applications, Ser. Nos. 07/948,524 and 07/948,522, whose contents are herein incorporated by reference, describe the construction and method of manufacture on a common substrate of individually addressable quantum well (QW) lasers that can be caused to oscillate in the transverse electric (TE) or in the transverse magnetic (TM) mode, and QW laser constructions that can be switched from oscillating in the TE polarization mode to the TM polarization mode, or vice versa. This has been accomplished in certain material systems by controlling the type of strain induced in the epitaxially deposited active region due to lattice mismatches with the substrate. Thus, in most material systems allowing heavy hole and light hole transition, when the N=1 heavy hole is the lowest energy state and therefore the state whose population is most easily inverted, usually true for unstrained and compressively strained III-V alloy systems, TE polarization gain will predominate. However, by reversing the light hole and heavy hole band edges, achieved in certain material systems by inducing tensile strain into the active region, TM polarization gain will predominate. In the degenerate condition, where the light hole and heavy hole bands are substantially coincident in energy, the polarization of the emission can be determined by threshold carrier density and other factors, such as temperature, facet reflectivity, cavity length and intracavity optical loss.
In general, the desired polarization mode emitter can be achieved with either a single QW, carefully adjusted, or separate QWs for TE and TM mode gain, respectively, with the polarization mode of laser oscillation dependent upon the modal gain characteristics and the threshold gain. The necessary gain characteristic has one polarization with lowest transparency current, and the orthogonal polarization with a greater peak gain. For some range of active region parameters (thickness, composition, placement within the confining region, etc.), these characteristics can be obtained, and so the polarization will be determined by the threshold gain. Therefore, the polarization of each device can be selected, for example by introducing an additional loss into one of the devices, thereby forcing it to oscillate in the higher-gain polarization. On the other hand, a device without this additional loss will simply oscillate in the polarization which has lowest transparency current. The additional loss could be provided by an unpumped section along the cavity, low mirror reflectivity, shorter cavity, etc. Similarly, the polarization of each device could be switched, by using an intracavity loss modulator.
This polarization selectivity mechanism is demonstrated by the polarization-dependent gain-current characteristics shown in FIG. 1, in which modal gain, g, for both TM and TE modes is plotted along the ordinate, and current, I, for both modes is plotted along the abscissa. The curve 10 labelled TE shows the gain characteristic for the TE-mode, and the curve 11 labelled TM that for the TM-mode. When the operating conditions are represented by the vertical line 13, to the left of the crossover 14, then the TE-gain is higher and TE-polarized light is emitted. When the operating conditions are represented by the vertical line 15, to the right of the crossover 14, then the TM-gain is higher and TM-polarized light is emitted.
Applying the principles described above to the material systems disclosed in the referenced prior applications resulted in laser devices that could emit in the spectral range of about 600-650 nm. However, there are important applications for laser devices that emit significantly beyond the 650 nm limit.